Using a case study from the island of Elba, Italy, we seek to test the hypothesis that
the presence of minerals with low frictional strengths can explain prolonged slip on
low-angle normal faults. The central core of the Zuccale low-angle normal fault
contains a distinctive fault rock zonation that developed during progressive exhumation.
Most fault rock components preserve microstructural evidence for having accommodated
deformation entirely, or partly, by frictional mechanisms. One millimeter thick sample
powders of all the major fault rock components were deformed in a triaxial deformation
apparatus under water-saturated conditions, at room temperature, and at constant effective
normal stresses of 25, 50, and 75 MPa. Pore fluid pressure was maintained at 50 MPa
throughout. Overall, the coefficient of friction (m) of the fault rocks varies between
0.25 and 0.8, emphasizing the marked strength heterogeneity that may exist within
natural fault zones. Also, m is strongly dependent on fault rock mineralogy and is
<0.45 for fault rocks containing talc, chlorite, and kaolinite and >0.6 for fault rocks
dominated by quartz, dolomite, calcite, and amphibole. Localization of frictional slip
within talc-rich portions of the fault core can potentially explain movements along the
Zuccale fault over a wide range of depths within the upper crust, although the
mechanical importance of the talc-bearing fault rocks likely decreased following their
dismemberment into a series of poorly connected fault rock lenses. Additionally, slip
within clay-bearing fault gouges with m between 0.4 and 0.5 may have facilitated
movements in the uppermost (<2 km) crust. For several other fault rock components,
m varies between 0.5 and 0.8, and mineralogical weakening alone is insufficient to
account for low-angle slip. In the latter fault rock components, other weakening
mechanisms such as the development of high fluid pressures, or dissolution-precipitation
creep, may have been particularly important in reducing fault strength.